1. Field of the Invention
The invention relates to centrifugal clutches and more particularly, to a centrifugal clutch construction in which the driving and driven members are coupled together instantaneously upon the driving member reaching a predetermined speed, which engagement speed is higher than the disengagement speed of the coupled members. More particularly, the invention relates to a centrifugal clutch construction in which the release speed of the clutch shoes is accurately controlled and the retaining stresses are evenly distributed by providing a conical surface-to-conical surface retaining clamping action.
2. Description of the Prior Art
There are numerous types and arrangements of centrifugal clutch constructions in which one or more clutch shoes are retained in retracted position until sufficient centrifugal forces act on the shoes to overcome a retaining spring whereupon the clutch shoes move outwardly and engage a stationary clutch drum. Such clutch constructions experience excessive wear of the clutch shoes linings since the coupling engagement is gradual as the driving member comes up to engagement speed resulting in undesirable slippage and heating between the shoe lining material and driven member.
Various centrifugal clutch constructions have been developed to provide a more sudden or instantaneous engagement of the clutch shoes with the clutch drum to eliminate such undesirable wear and heating. Many of these delayed action clutch constructions use spring means to retain the clutch shoes in retracted position and to actuate the rapid engagement. Examples of such clutch constructions are shown in U.S. Pat. Nos. 1,737,825, 2,000,713, 3,367,463, and 3,752,285.
Other clutch constructions use other delay engagement means, such as a fluid dashpot as shown in U.S. Pat. No. 3,850,276, while still other clutch constructions use spring means to achieve an instantaneous disengagement of the clutch shoes from the driven member as opposed to instantaneous engagement. Examples of these clutch constructions are shown in U.S. Pat. Nos. 2,534,133, 2,755,902, and 3,367,463.
Still other clutch constructions use spring-biased detent balls to retain the clutch shoes in retracted position. In these constructions the clutch shoes are held in clamped position by the spring biased balls requiring the centrifugal force exerted on the shoes to be sufficiently great to overcome this clamping retaining force of the spring ball detents. This clamping force generally is assisted by a continuous garter spring which extends about the outer circumference of the clutch shoes and which is used primarily for return of the shoes from extended to retraction position.
The ball detents which generally have a spherical shape, are clampingly engaged with the clutch shoes within countersunk recesses in the shoes or a projection formed on the shoes. The contact area between the spherical ball detents and the clutch shoes is generally along a straight line or at a point which will occur when a spherical surface (detent ball) contacts a flat projection or countersunk conical surface. This straight line or point contact area and resulting contact angle is difficult to determine for such clutch constructions. Thus, such relationships and characteristics make it difficult to accurately design a centrifugal clutch construction for engagement at predetermined speeds, since the release speed of the shoes from such spring-biased ball detents is largely dependent upon the contact angle between these components which, as set forth above, is difficult to determine. Such straight line or point contact areas, also present problems in such clutch constructions due to the great amount of stress exerted on such small areas, which results in undesirable breakage, wear and maintenance problems.
Furthermore, the use of springs or spring biased members in which the spring itself provides the main clamping force on the clutch shoes, presents design and operating problems in that spring characteristics are affected by change in ambient temperature, age, wear, and the like.
There is no known construction of which I am aware which provides an instantaneously engageable clutch construction which engages at a higher speed than its disengagement speed, which uses spring biased members to retain the clutch shoes in retracted position until sufficient centrifugal force is developed in the shoes, and in which the clutch shoe clamping engagement is through a conical surface-to-conical surface contact area which enables the contact angle between the clamping members to be accurately determined, thereby enabling the release speed to be accurately determined, and which distributes the contact stress between the clamping members over a greater area than in known clutch constructions.